1. Field of the Invention
The invention relates to a method for manufacturing an anilox roller, in particular for flexographic printing machines, the roller comprising a cylindrical core and a sleeve which is detachably held on the core and has at its surface an anilox layer with a grid pattern of pits. The invention further relates to an anilox roller manufactured according to this method and to a method of recycling the same.
2. Description of the Related Art
In flexographic printing, the printing ink is applied onto an impression cylinder by means of an anilox roller. The surface of the anilox roller passes through a chamber-type doctor assembly in which the minute pits of the grid pattern are filled with ink. When, then, the anilox roller comes into contact with the impression cylinder at another location of its circumference, the ink is transferred onto the printing parts of the impression cylinder.
Known anilox rollers typically have a non-deformable cylindrical metal core which is provided with bearing studs at both ends or is detachably mounted on a continues axle, so that it may be supported in a frame of a printing press and may be driven for rotation. In the manufacturing process, the surface of the cylindrical core is provided with a primer layer, and the anilox layer consisting of a ceramic material is then applied directly on the primer layer. Finally, the individual pits are formed in the surface of the anilox layer by means of a laser.
When, after long-term use, the anilox roller shall be reprocessed, because the anilox layer has become damaged or worn, the whole anilox layer and, as the case may be, also the primer layer must be ground away before, after renewed priming, a new anilox layer may be applied and may be provided with the pit pattern by laser processing. This recycling procedure is very cumbersome and expensive.
EP 1 132 209 A discloses an anilox roller in which the cylindrical core is not made of metal but of a synthetic resin reinforced by carbon fibers, so that it has a lower weight at equal stability. Although this facilitates the handling of the anilox roller and improves the smoothness of running, the reprocessing is cumbersome, similarly as in case of a metal anilox roller.
On the other hand, anilox rollers of the type indicated in the opening paragraph of the description are known, which are based on the so-called “sleeve technology”. In these anilox rollers, the anilox layer is not applied directly on the cylindrical core but on a hollow cylindrical sleeve which is then thrust onto the cylindrical core. With this technology, it is possible to withdraw the sleeve from the core and to replace it with a new sleeve.
Frequently, the core has a compressed air system with which it is possible to discharge compressed air at locations distributed over the surface of the core, thereby to widen the sleeve, so that it may be thrust on and drawn off more easily. However, this procedure requires a specific and relatively complex construction of the sleeve. When exposed to compressed air, it is desired that only the internal diameter of the sleeve becomes larger, whereas the external diameter should remain unchanged as far as possible, because otherwise the anilox roller made of a ceramic material could become cracked and/or could burst off. Consequently, the sleeve must have, on the inner side of a stiff layer which retains its shape as far as possible and is formed for example by synthetic resin reinforced with glass fibers, a compressible layer which is compressed when the internal diameter is widened. In order for the pneumatic pressure to be evenly distributed on the inner surface of the sleeve, a very thin internal layer of a stiffer material should again be provided below the compressible layer.
In order to achieve a good print quality, it is required that the sleeve has excellent properties in terms of smoothness of running. However, this requirement is difficult to fulfill for sleeves with the construction described above, because, due to the presence of the compressible layer, the stiff outer layer is no longer directly supported on the rigid core. The outer layer must therefore have a high stiffness in itself. This can only be achieved by correspondingly large layer thicknesses, resulting in increased material consumption and costs. Particularly in case of large printing widths, the large wall thickness of the sleeve, for example in the order of 25 mm or more, makes the handling of the sleeve more difficult and increases the mass of inertia of the sleeve hand hence the risk of an imbalance, so that the required smoothness of running is difficult to achieve. Also, the large wall thickness of the sleeve makes it more difficult to comply with the limits for the external diameter of the anilox roller and for the internal diameter thereof, i.e. for the external diameter of the core. The external diameter of the anilox roller must be compatible with the conditions for building-in the roller in the inking unit of the printing press, and limitations regarding the printing process, e.g. drying time for the ink, centrifugal forces at the periphery of the anilox roller, construction of the doctor assembly, etc. must also be taken into consideration. On the other hand, a reduction of the internal diameter leads to increased costs for the core which must then fulfill the same stability requirements with a smaller external diameter.
It is another drawback of the sleeve technology that the anilox layer at the ends of the sleeve is likely to be damaged when a sleeve is thrust onto the core.
When the anilox layer is damaged or worn, the exchange of the whole sleeve is not economical, because of the relatively high costs for the sleeve. Removing the old anilox layer from the sleeve and building a new anilox layer is cumbersome, similarly as in case of conventional anilox rollers in which the anilox layer is formed directly on the core.